A widely used type of fuel quantity gauge for monitoring the level of fuel in a tank such as the fuel tank of an aircraft is the capacitance type gauge. One type of such a capacitance type fuel gauge system is shown in U.S. Pat. No. 3,747,407 entitled Probe Mounted Electronics for Fuel Gauge System, issued July 24, 1973 to Irwin Wallman. In such a capacitance type system, a probe capacitor is mounted in the fuel tank with signal generating means such as an oscillator connected to the capacitor to provide an output signal having a magnitude which varies in accordance with the variation of the capacitance of the probe capacitor in accordance with the level of fuel in the tank. This output signal is processed in suitable circuitry for indicating such fuel quantity in the tank.
In the case of a DC type tank unit, a pair of oppositely poled diodes are connected to the output of the probe capacitor to provide a DC output signal which is fed to a remotely located signal processing circuitry for providing a readout of fuel quantity.
One of the problems in such a capacitative type signal is the interference to such electronic circuitry caused by high electromagnetic fields in the fuel tank environment. Typically, in electronic circuits filter capacitors are used to bypass an interfering signal to "ground". However, in fuel quantity gauge systems of the type to which this invention is directed, there is no ground to which the interference can be bypassed. In an aircraft fuel tank utilizing such a system, bypassing to the aircraft is not possible since this would create an entirely new set of problems. Previous attempts in the field of aircraft fuel tanks to deal with high electromagnetic fields have utilized a third tube at ground potential with three bypass capacitors rather than two, by the use of shielded wires and the like.